The invention is in the field of electrical switches, in particular, electrical switches relying on liquid gallium metal or liquid gallium alloy as a bridging electrical conducting material.
Electrical tilt switches and sensors such as thermostats, float controls, solenoids, relays, etc. are commonly used in a variety of electrical applications. The making or breaking of electrical contact in these switches, hence the electrical switching action, is generally accomplished by mechanical movement or tilting of the switch which causes a quantity of a bridging conducting material, commonly liquid mercury metal, contained therein to flow from one location to another. In a typical switch application, liquid mercury is positioned inside a housing into which a pair of spaced electrodes or electrical contacts extend. Depending on the physical orientation of the housing, the liquid mercury can provide a conductive pathway between the electrodes or be positioned such that there is an open circuit between the electrodes. The switch is closed and electrical contact made when the switch housing is moved in a manner such that the quantity of mercury flows toward and collects in the switch housing at a location where the mercury bridges the spaced electrodes. Conversely, the switch is opened and electrical contact broken when the switch housing is moved in a manner such that the quantity of mercury flows towards and collects at a different position in the switch housing out of contact with at least one of the electrodes.
In some configurations of these kinds of electrical switches, one electrode remains in continuous contact with the quantity of mercury and the electrical circuit is closed when the mercury contacts the other electrode. In these electrical switch configurations, the electrode that continuously contacts the mercury is referred to as the common electrode, while the electrode that intermittently contacts the mercury as a result of changes in the switch orientation is referred to as the arcing electrode because it is subjected to electrical arcing whenever the electrical circuit is made or broken.
In yet other configurations, there may be more than two electrodes within the switch housing such that more than one electrical circuit may be closed depending on the location of the quantity of mercury, hence the orientation of the switch.
The foregoing configuration, as well as other configurations of mercury-based electrical switches, and their applications, would be known to persons skilled in the art.
A problem with mercury-based electrical switches is the mercury is toxic to humans and animals, and exposure to mercury is a significant concern in any application or process in which it is used. Utilization of mercury during manufacturing may present a health hazard to plant personnel, and the disposal of devices that contain mercury switches or the accidental breakage of mercury switches during use may present indirect hazard to people within the immediate vicinity of the switch.
As a result of the toxicity of mercury, non-toxic replacements for mercury in electrical switch applications have been sought. A candidate for replacing mercury in electrical switches is liquid gallium metal or liquid gallium alloys. For example, U.S. Pat. No. 3,462,573 (Rabinowitz et al.) discloses vacuum type circuit interrupters using a pair of fixed electrodes, or contacts, having liquid gallium metal or liquid gallium alloys as the bridging conducting material between the electrodes or contacts. U.S. Pat. No. 5,391,846 (Taylor et al.) discloses an alloy substitute for mercury in switch applications comprising a gallium-indium-tin eutectic alloy which has been cleaned to remove all oxides. U.S. Pat. No. 5,792,236 (Taylor et al.) discloses a non-toxic liquid metal composition comprising gallium metal or gallium alloy for use as a mercury substitute in, among other things, electrical switches. U.S. Pat. No. 5,478,978 (Taylor et al.) discloses electrical switches and sensors which use a non-toxic liquid metal composition, namely gallium metal or gallium alloy. Each of the above references is hereby incorporated by reference.
The electrodes or electrical contacts in mercury-based switches are commonly molybdenum, tungsten or platinum. However, when gallium is used in electrical switches in place of mercury, it has been found that electrodes comprised of these materials deteriorate relatively rapidly. For example, our experiments show that molybdenum electrodes, tungsten electrodes and platinum electrodes have been found surprisingly to recess or erode rapidly after a relatively low number of arc cycles, which may be due to an interaction between gallium and these metals. As a result, our experience indicates that electrodes composed of platinum, tungsten or molybdenum are generally unsuitable for gallium-based switch applications.
Electrodes in mercury-based switches are also commonly composed of nickel, chromium or iron. Our experience indicates that thinning of the electrodes comprising either one of these metals may result from immersion in gallium, but the process may be slow if the operational temperature of the gallium-based switch is kept low. Consequently, these metals may be used as common electrodes in gallium-based switches. However, it has been found that each of these metals are unsuitable as the arcing electrode in gallium-based electrical switches since each may be eroded by the harsh conditions created by the electrical arc that results whenever an electrical circuit is made or broken. Consequently, high temperature arc operations may result in these electrodes melting away in gallium-based switches.
In view of the above drawbacks and problems, it is apparent that a need exists for a novel approach to gallium-based electrical switch construction which will reduce or eliminate such drawbacks and problems.
An object of the present invention is to provide an improved gallium-based electrical switch.
In accordance with the present invention there is provided an electrical switch comprising a housing defining a cavity, at least two spaced electrodes, each electrode extending through the housing into the cavity, and wherein at least one of said at least two electrodes is composed of tantalum or a tantalum alloy, and a moveable amount of liquid gallium or liquid gallium alloy within the cavity to electrically connect and disconnect any two of said at least two electrodes as a result of movement of the housing.
An advantage of the present invention is providing a gallium-based electrical switch in which the electrodes withstand degradation by the gallium or the electrical arc to a better degree than the electrodes in gallium-based switches in the prior art.